package/openldap: security bump to version 2.4.56

package/openldap: security bump to version 2.4.56

Fixes the following security issue:

- CVE-2020-25692: A NULL pointer dereference was found in OpenLDAP server
  and was fixed in openldap 2.4.55, during a request for renaming RDNs.  An
  unauthenticated attacker could remotely crash the slapd process by sending
  a specially crafted request, causing a Denial of Service.

- CVE-2020-25709: Assertion failure in CSN normalization with invalid input

- CVE-2020-25710: Assertion failure in CSN normalization with invalid input

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
[Peter: add CVE info]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>